To understand the effect of low temperature heat treatment on microstructure of nodular graphite, nodular cast irons with the same chemical composition but different microstructure were made by the heredity of raw materials. The effects of low temperature heat treatment on the different microstructures of graphite were studied. The results show that after the low temperature treatment, the modularization rate and level are improved observably for as-cast graphite with lower degree of graphitization and worse effect of nodularization and the mean sphere diameter of the graphite increases; the spot graphite arises for as-cast graphite with higher degree of graphitization and better effect of nodularization, the mean sphere diameter of the graphite decreases, the spot and nodular graphite distribute alternately, the spheroidization grade and nodularization rate do not change obviously.